2 Contents What is Premature Yeast Flocculation (PYF)?Development of an assay to predict the PYF status of maltsThe importance of yeast strain on the severity of PYFInvestigating the “antimicrobial peptide hypothesis”Conclusions

3 What is Premature Yeast Flocculation (PYF)?Early or abnormally heavy onset of flocculationlow suspended cell countsincomplete fermentation of sugars to alcoholSporadic problem in brewery fermentationsregion and harvest conditions dependentInduced by factor(s) which have been shown to originate from malt (van Nierop et al., 2004)

4 Why PYF is Important? Incomplete conversion of sugars to alcoholFlavour abnormalities (e.g. diacetyl)Potential issues with the re-use of the yeastRequirement to blendSignificant financial and logistical problemsThe uptake of diacetyl by yeast: an important aspect of flavour maturation in lager beers

6 Predictive tests for PYFStandard malt analysis is unable to predict PYFLaboratory-scale fermentation assays are the most widely adoptedFermentation tests take several days to be completedMonitor gravity and measure suspended yeast cellsSome rapid and micro-scale tests have been developed

12 Lager strains differ in susceptibility to PYF (3)Fermentation Profiles using different PYF+ve Worts and the ‘Industrial’ Lager YeastPanteloglou, A.G., Smart, K.A., and Cook, D.J. The importance of wort nutrients and yeast strain on the incidence of premature yeast flocculation. In preparation.

24 ConclusionsYeast strain is a factor in the severity of PYF which is presented in a breweryLager yeasts have different degrees of susceptibility even to the same PYF factor(s)The more flocculent yeast strain SMA exhibited a higher degree of susceptibility than the less flocculent yeast strain W34/70The fermentation performance for a PYF+ve wort could be improved by using a less flocculent yeast which is less sensitive to PYF

25 Conclusions (2): Mini-FV experimentsMany markers of fermentation performance and nutrient uptake were ‘normal’ in the PYF+ve fermentationsIn this experiment the PYF factor(s) did not appear to unduly influence nutrient uptake, inhibit yeast growth or cause the death of the cellsPYF+ve sample probably represented the ‘bridging polysaccharide’ type of PYF rather than the ‘antimicrobial peptide’

26 AcknowledgmentsThis research was financed by the UK Home Grown Cereal Authority (HGCA) and the University of NottinghamAdditional thanks to my colleagues in The University of Nottingham Brewing Science Group for their help, support and advice